A Study of High Energy Emission from the TeV blazar Mrk 501 during Multiwavelength Observations in 1996

We present the results of a multiwavelength campaign for Mrk 501 performed in March 1996 with ASCA, EGRET , Whipple, and optical telescopes. The X–ray flux observed with ASCA was 5 times higher than the quiescent level and gradually decreased by a factor of 2 during the observation in March 1996. In the X–ray band, a spectral break was observed around 2 keV. We report here for the first time the detection of high-energy γ–ray flux from Mrk 501 with EGRET with 3.5 σ significance (E > 100 MeV). Higher flux was also observed in April/May 1996, with 4.0 σ significance for E > 100 MeV, and 5.2 σ significance for E > 500 MeV. The γ–ray spectrum was measured to be flatter than most of the γ–ray blazars. We find that the multiband spectrum in 1996 is consistent with that calculated from a one-zone SSC model where X–rays are produced via synchrotron emission, and γ-rays via inverse Compton scattering of synchrotron photons in a homogeneous region. The flux of TeV γ–rays is consistent with the predictions of the model if the decrease of the Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Astronomy Department, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, P. O. Box 97, Amado, AZ 85645-0097 The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo, Japan 192-0397 Laboratory for High Energy Astrophysics, NASA/GSFC, Greenbelt, MD 20771 Dept. of Astronomy, Univ. of Maryland, College Park, MD 20742 Universities Space Research Association Landessternwarte Heidelberg, Kőnigstuhl, D-69117 Heidelberg, Germany